[LeetCode] 102. Binary Tree Level Order Traversal

Given a binary tree, return the level order traversal of its nodes’ values. (ie, from left to right, level by level).

For example:
Given binary tree [3,9,20,null,null,15,7],

    3   / \  9  20    /  \   15   7

return its level order traversal as:

[  [3],  [9,20],  [15,7]]

class Solution {public:    vector<vector<int>> levelOrder(TreeNode* root) {        if (root == nullptr) return {};        vector<vector<int>> res;        vector<int> level;        queue<TreeNode *> q;        q.push(root);        q.push(nullptr);        while (!q.empty()) {            TreeNode *ptn = q.front();            q.pop();            if (ptn == nullptr) {                res.push_back(level);                level.clear();                if (!q.empty())                    q.push(nullptr);                continue;            }            level.push_back(ptn->val);            if (ptn->left)                q.push(ptn->left);            if (ptn->right)                q.push(ptn->right);        }        return res;    }};